1. Field of the Invention
The present invention relates to ceramics having excellent high-frequency characteristics (hereinafter often called high-frequency ceramics) and, particularly, to high-frequency ceramics favorably used as high-frequency wiring boards and insulating substrates that can be co-fired with a low-resistance conductor such as copper or silver, and that are used for a variety kinds of devices for transmitting high-frequency signals of microwaves or millimeter waves, such as semiconductor element-containing packages, dielectric resonators, LC filters, capacitors, dielectric waveguides and dielectric antennas.
2. Description of the Prior Art
As a ceramic multi-layer wiring board, there has heretofore been used the one in which a wiring layer of a high-melting metal such as tungsten or molybdenum is formed on the surface of or inside of the insulating substrate of an aluminous sintered product. In recent years facing the era of information technology, further, the frequency band that is used is shifting toward higher frequencies.
In such high-frequency wiring boards, it is required that the conductor forming a wiring layer has a small resistance from the standpoint of transmitting high-frequency signals without loss and that the dielectric loss of the insulating substrate is small in the high-frequency region.
However, the existing high-melting metals such as tungsten(W) and molybdenum (Mo) have large resistances, transmit signals at slow speeds, and make it difficult to transmit signals in high-frequency regions. For the high-frequency wiring boards that treat high-frequency signals in a millimeter region of not lower than, for example, 30 GHz, the above-mentioned high-melting metals cannot be used. It therefore becomes necessary to use a low-resistance metal such as copper, silver or gold to substitute for the high-melting metals.
However, the low-resistance metals have low melting points and cannot be co-fired with the ceramics such as alumina. Therefore, there has been developed a wiring board using, as an insulating substrate, glass ceramics which is a composite material of a glass and ceramics.
Japanese Unexamined Patent Publication (Kokai) No. 240135/1985 proposes a multi-layer wiring board obtained by co-firing a zinc borosilicate glass to which are added such fillers as Al2O3, zirconia and mullite, together with a low-resistance metal. Further, Japanese Unexamined Patent Publication (Kokai) No. 298919/1993 proposes a glass ceramic material in which mullite and cordierite are precipitated as crystal phases.
On the other hand, to the wiring board are connected various electronic parts and the input/output terminals depending upon the applications. Therefore, the wiring board is often cracked or damaged due to stress exerted in the step of connection. To prevent such inconvenience, the insulating substrate in the wiring board must have a high mechanical strength.
The present applicant therefore has proposed in a previously filed Japanese Unexamined Patent Publication (Kokai) No. 275963/1998 ceramics having an excellent mechanical strength and excellent high-frequency characteristics. The ceramics contains a spinel crystal phase containing ZnO and Al2O3, an SiO2 crystal phase and an enstatite crystal phase, and contains an amorphous phase comprising substantially SiO2 or SiO2 and B2O3, has a large mechanical strength, a small dielectric loss in the high-frequency band, and is very useful as an insulating substrate for a high-frequency wiring board. The above ceramics is obtained by preparing a slurry by adding an organic binder to a mixture of starting materials of a ZnO powder and an amorphous silica powder added to a glass powder that contains SiO2, Al2O3, MgO, ZnO and B2O3 at a particular ratio, forming a green sheet by using this slurry, and firing the obtained green sheet at 825 to 975xc2x0 C.
However, though the glass ceramics disclosed in Japanese Unexamined Patent Publications (Kokai) Nos. 240135/1985 and 298919/1993 can be co-fired together with a low-resistance metal such as copper, silver or gold, they, generally, exhibit large dielectric losses in a high-frequency band and low mechanical strengths. For example, the above glass ceramic sintered product has a strength of about 15 kg/mm2 which is very smaller than that of the aluminous sintered product.
On the other hand, the ceramics disclosed in Japanese Unexamined Patent Publication (Kokai) No. 275963/1998 exhibits a large mechanical strength but has a problem in that it expands at high temperatures (e.g., has the coefficient of thermal expansion of larger than 8 ppm/xc2x0 C). That is, a large difference in the thermal expansion is exhibited from the high-frequency elements (having coefficients of thermal expansion of 5 to 8 ppm/xc2x0 C.) such as GaAs chip and SiGe chip used in high-frequency bands such as of microwaves and millimeter waves. Therefore, the wiring board equipped with the insulating substrate formed of the above ceramics produces a thermal stress on the interface between the high-frequency element and the insulating board due to heat generated during the operation of the high-frequency element and at the time of mounting the high-frequency element, resulting in the occurrence of cracks. Besides, the above ceramics is not fully satisfactory from the standpoint of dielectric loss in a high-frequency region.
It is therefore an object of the present invention to provide high-frequency ceramics that can be produced by firing at not higher than 1000xc2x0 C., that can be co-fired with a low-resistance conductor of gold, silver or copper, that exhibits a large strength, and that exhibits a small dielectric loss even in a high-frequency region, and a method of producing the same.
It is another object of the present invention to provide a high-frequency wiring board equipped with an insulating substrate formed of the above high-frequency ceramics.
The present inventors have closely studied the ceramics disclosed in Japanese Unexamined Patent Publication (Kokai) No. 275963/1998 proposed earlier by the present applicant, have discovered a novel fact that the ceramics exhibits a large coefficient of thermal expansion and a large dielectric loss in a high-frequency region (e.g., 60 GHz) since SiO2 crystals such as quartz are precipitated in large amounts (e.g., not smaller than 10% by weight) and that the amount of SiO2 crystals that are precipitated vary depending upon the amount of impurity metals contained in the amorphous silica starting material, and have completed the invention.
That is, the present invention provides ceramics having excellent high-frequency characteristics which contain SiO2, Al2O3, MgO, ZnO and B2O3 as constituent components, said ceramics comprising:
30 to 50% by weight of a crystal phase containing ZnO and Al2O3;
5 to 15% by weight of a crystal phase containing SiO2 and MgO; and
40 to 60% by weight of an amorphous phase comprising substantially SiO2 or SiO2 and B2O3;
wherein a content of an SiO2 crystal phase is suppressed to be not larger than 6% by weight, and the dielectric loss at 60 GHz is not larger than 15xc3x9710xe2x88x924.
The invention further provides a method of producing ceramics having excellent high-frequency characteristics, comprising:
preparing a mixture of starting materials of 65 to 85% by weight of a crystallized glass containing SiO2, Al2O3, MgO, ZnO and B2O3, 5 to 20% by weight of a ZnO powder, and 1 to 20% by weight of an amorphous silica powder containing not larger than 500 ppm of impurities calculated as metals;
preparing a slurry by adding an organic binder to said mixture;
molding said slurry; and
removing the binder from the obtained molded article, followed by firing at 800 to 1000xc2x0 C.
The invention further provides a high-frequency wiring board comprising an insulating substrate formed of the above-mentioned ceramics, and a wiring layer formed on the surface of and/or inside of the insulating substrate and is capable of transmitting signals of high frequencies of not lower than 1 GHz.
The present invention relates to an improvement in the ceramics disclosed in Japanese Unexamined Patent Publication (Kokai) No. 275963/1998. The high-frequency ceramics of the present invention is greatly different from the ceramics of the prior art with respect to that the content of the SiO2 crystals is suppressed to be not larger than 6% by weight. Upon suppressing the content of SiO2 crystals, the dielectric loss at 60 GHz is as low as 15xc3x9710xe2x88x924 as demonstrated by experimental examples appearing later, and the coefficient of thermal expansion at room temperature through up to 400xc2x0 C. is in a range of 5 to 8 ppm/xc2x0 C. which is close to that of high-frequency elements such as GaAs chip and SiGe chip. As a result, the high-frequency ceramics of the present invention can be very effectively used as an insulating substrate for the high-frequency wiring boards.
That is, like the ceramics of the above prior art, the high-frequency ceramics of the present invention is produced by using a mixed powder of a crystallized glass containing SiO2, Al2O3, MgO, ZnO and B2O3, a ZnO powder and an amorphous silica powder. Here, impurity metals such as aluminum, iron and antimony contained in the amorphous silica powder and other oxides of impurity metals, seriously affect the precipitation of SiO2 crystals such as quartz. Concretely speaking, when the amounts of impurities contained in the amorphous silica powder are great, the crystallization of amorphous silica is promoted during the firing, SiO2 crystals precipitate in large amounts, the obtained ceramics exhibits a large coefficient of thermal expansion and increased dielectric loss in the high-frequency regions. According to the present invention, use of a highly pure amorphous silica in which the content of impurities is suppressed to be not larger than 500 ppm calculated as metals, contributes to suppressing the crystallization of amorphous silica during the firing and makes it possible to obtain the ceramics in which the content of SiO2 crystals is suppressed to be not larger than 6% by weight.